Generally, communication signals are transmitted at a desired frequency that is obtained by multiplying the original information carrying base band signal by a carrier frequency. In a Radio Frequency (RF) transmitter, for example, the desired RF frequency is typically obtained from the information carrying digital base band signal by converting the digital signal to an analog signal, and then mixing the analog signal with an RF carrier frequency signal using one or more mixers.
A number of direct synthesis techniques have been proposed or suggested for directly synthesizing RF signals from the information carrying digital base band signal. Among other benefits, direct synthesis techniques exhibit reduced footprint and power consumption characteristics. For example, high speed digital-to-analog converters (DACs) and delta-sigma converters have been proposed for this task. Direct synthesis techniques that employ high speed DACs typically exhibit a signal-to-noise ratio that is limited by the precision of the high speed DACs, such as high resolution and high speed current steering DACs. For example, existing high speed DACs typically have a precision of less than 8 bits when operating at the required multi Giga-sample per second rates.
Likewise, direct synthesis techniques that employ delta-sigma converters exhibit narrow signal bandwidths relative to the desired RF frequency. Typically, existing techniques employ delta-sigma converters having a relatively low order (e.g., up to an order of four) and require high over-sampling ratios (e.g., an over-sampling ratio of 64 or more). To alleviate this problem, multi-bit delta-sigma quantizers have been proposed. While this improvement may reduce the requirement in terms of over-sampling ratio and noise shaping, multi-bit DACs with high accuracy are required after the quantizer and the multi-bit delta-sigma quantizers have difficulty achieving the desired Spurious Free Dynamic Ranges (SFDRs) of 90 dB or more
A need therefore exists for improved methods and apparatus for direct synthesis of RF signals using a delta-sigma modulator. A further need exists for direct synthesis techniques that exhibit improved signal bandwidth, signal-to-noise ratios and filtering requirements for out-of-band noise.